Put the onion, celery, garlic, bay leaf, carrots, and leek in large saucepan and add the stock. Bring to a boil and simmer for 15-20 minutes, or until the vegetables are very soft; stir occasionally. Remove from heat and cool for 5 minutes. Carefully transfer to a food processor or use a hand blender to blend until smooth. Return the soup to the pan and heat through gently, stirring regularly. Sprinkle with kelp flakes and serve hot.